* The Decibel *

**
The ***decibel (dB)* is the unit of relative power. If one signal
is twice as loud as another, it has a gain of 3dB. One advantage of
expressing power in decibels is that, in a multi-stage amplifier,
the power gains of the stages can simply be added together.
The general expression for calculating the decibel gain of a circuit
(or an antenna) is:-

*Enter values for 'Power Out' and 'Power In' and press the
' = ' button.*

## dBW

The UK amateur radio licence expresses the maximum permitted output
power for each band in dBW. This is the
output power, in dB relative to 1 Watt. To express this in Watts,
the following formula can be used:-

## Antenna Gain

The gain of an antenna is relative to either a theoretical, *
isotropic* aerial (dBi) or relative to a half-wave dipole (dBd).
For any given antenna, the isotropic gain is 2.1 dB greater than the
dBd figure so...
dBd = dBi - 2.1 or...
dBi = dBd + 2.1

## Voltage Gain

The dB gain of one *voltage* relative to another can be
calculated with the formula:-

However, the result is only correct when the input and output impedances
of the circuit are the same.

**
73 from John G4VWL @GB7DEE email: john@vwlowen.demon.co.uk
**

*This page was found on Packet Radio. If you find it useful send thanks to John (G4VWL)*